(1) Field of the Invention
The present invention relates to a solid-state imaging device including column amplifier units that are each provided for a different one of column signal lines which carry pixel signals from a plurality of pixel units arranged in a matrix.
(2) Description of the Related Art
Imaging equipment that uses a solid-state imaging device, such as a home video camera and a digital still camera, has become widely used in recent years. Such imaging equipment may include an amplifier-type image sensor as a solid-state imaging device.
FIG. 1 is a block diagram showing a structure of a solid-state imaging device according to a conventional technique. This solid-state imaging device is a MOS image sensor, and includes an imaging area 510 made up of a plurality of pixel units 501 arranged in a matrix, a load circuit 502, a row selection decoder 503, a column selection decoder 504, a column amplifier unit 505 made up of a plurality of column amplifiers 506, and an output amplifier 507. Each column signal line carries a pixel signal of a pixel unit in a row selected by the row selection decoder 503, from among pixel signals of pixel units connected to a corresponding column. Each column amplifier 506 amplifies a pixel signal from a corresponding column signal line. A horizontal common signal line carries an amplified signal selected by the column selection decoder 504, from among amplified signals of the plurality of column amplifiers 506.
FIG. 2 is a block diagram showing one column amplifier 506. In FIG. 2, the column amplifier 506 includes a load circuit 511 and an amplifier circuit 512 that are connected in series with each other. The load circuit 511 and the amplifier circuit 512 are each formed by one MOS transistor. The amplifier circuit 512 inverse-amplifies a pixel signal from a column signal line, and outputs the inverse-amplified signal.
Such an amplifier-type solid-state imaging device exhibits excellent characteristics such as low noise. However, the amplifier-type solid-state imaging device also has the following problem. When strong light enters, noise called horizontal streaking or horizontal banding appears in a horizontal direction of an image. This phenomenon occurs because a large change of an amplified signal outputted from a column amplifier 506 corresponding to strong light affects adjacent column amplifiers 506. The noise is called horizontal streaking in the case where it is narrow in a vertical direction, and horizontal banding in the case where it is wide in the vertical direction.
FIGS. 3A and 3B show an example image where horizontal streaking appears. The image shown in FIG. 3A is obtained, for instance, when a wall having a small, very bright window is photographed from inside a dark room. The white rectangular area in the middle of the image corresponds to the bright window. Strong light enters a rectangular area in the imaging area 510 corresponding to this white rectangular area, whilst weak light enters a surrounding area of the rectangular area. As shown in FIG. 3A, dark horizontal streaking appears on both sides of the rectangular area.
FIG. 3B shows pixel brightness in A-A′ and B-B′ shown in FIG. 3A. In FIG. 3B, the vertical axis represents pixel brightness, and the horizontal axis represents a pixel position in the horizontal direction. As can be seen from FIG. 3B, horizontal streaking or horizontal banding contains two types of noise, namely, highlight horizontal streak noise and highlight horizontal streak shading. The difference between A-A′ and B-B′ in FIG. 3B is the highlight horizontal streak noise. When the wall has a uniform color, the image other than the highlight rectangular area should have the same brightness. Despite this, B-B′ is darker than A-A′. The difference in brightness in the part of B-B′ corresponding to the wall in FIG. 3B is the highlight horizontal streak shading. Although the part of B-B′ corresponding to the wall should have the same brightness, the brightness differs between a vicinity of the highlight rectangular area and edges of the image.
Such horizontal streaking can occur due to the following reason. Since an extremely large number of column amplifiers 506 exist in the column amplifier unit 505, parasitic resistances of a power line and a ground line which connect these numerous column amplifiers 506 are not negligible. This being so, a current flowing in each column amplifier 506 causes a difference in potential between middle and edge portions of the column amplifier unit 505. Moreover, when a current flowing in a column amplifier 506 fluctuates due to strong light, fluctuations in the power line and the ground line affect other column amplifiers 506, as a result of which horizontal streaking emerges.
Various techniques have been proposed to prevent such horizontal streaking or horizontal banding (for example, see Patent Reference 1).
FIG. 4 is a block diagram showing a structure of a column amplifier included in a solid-state imaging device disclosed in Patent Reference 1. A rough structure of this solid-state imaging device is the same as the one shown in FIG. 1, where the column amplifier shown in FIG. 4 corresponds to one of the column amplifiers provided on a column basis in the column amplifier unit 505 in FIG. 1. The column amplifier shown in FIG. 4 includes a constant current source 512, an amplifier circuit 522, a limiter circuit 523, and a load circuit 524.
The limiter circuit 523 is connected to an output signal line of the amplifier circuit 522, and performs a switching operation of turning ON when an output voltage of the amplifier circuit 522 exceeds a threshold value. By limiting the output voltage of the amplifier circuit 522 in this way, current consumption of the amplifier circuit 522 is held constant to thereby prevent fluctuations in the power line and the ground line to which the plurality of column amplifiers are connected.    [Patent Reference 1] Japanese Patent Application No. 2004-058621, FIGS. 3 and 6    [Non-patent Reference 1] Kenji Taniguchi LSI sekkeishano tameno CMOS analog kairo nyumon (Introduction to CMOS analog circuits for LSI designers), CQ Publishing, January 2005
However, the circuit disclosed in Patent Reference 1 has the following problem.
The limiter circuit 523 performs the switching operation of turning ON when the amplified signal exceeds the threshold value. This switching operation induces a current fluctuation. Patent Reference 1 discloses the addition of a regulated cascode circuit to solve this problem. This, however, requires an increase in the number of circuit elements. Accordingly, the circuit disclosed in Patent Reference 1 is not suitable for pixel miniaturization and high-densification.